A seat belt retractor having the above-mentioned characteristics is described in DE 200 15 402 U1. In addition to a torsion bar disposed between the belt shaft and the profiled head thereof, this seat belt retractor has two further force limiting devices connected in series in the form of two friction couplings, which are coupled to one another via an externally and internally toothed ring. The first friction coupling having a high level of force limitation acts between the belt shaft and/or the profiled head thereof and the toothed ring, and the second friction coupling between the toothed ring and the housing. In the initial state, the toothed ring is fixed to the housing via a catch that can be controlled by a control device, wherein, if necessary, the catch is controlled disengaged from the toothed ring by the control device such that the toothed ring can be rotated freely in relation to the belt retractor housing.
In the known belt retractor, the control of the force limiting elements is configured such that the toothed ring is fixed to the housing via the catch so that the higher level of force limitation acts first in case of an accident. If a signal emitted by the vehicle determines that based on the driver's height, weight or position, or based on the severity of the accident, the restraining force acting on the occupants is becoming too large, the catch is controlled disengaged from the toothed ring so that the toothed ring can be rotated freely in relation to the housing. Thus, a relative movement occurs between the housing and the toothed ring, wherein the second friction coupling located between the housing and the toothed ring acts as a force limiting device with a lower level of force limitation. Accordingly, the switching operation is accomplished from the high level of force limitation to the lower level of force limitation and is not reversible. Insofar, the known belt retractor has the disadvantage that the high level of force limitation acts first also on lower-weight or buckled-up persons, resulting in an accordingly strong increase in the restraining force. The level of force limitation is only reduced when the restraining force is too high. A further disadvantage is that the known seat belt retractor does permit consideration of the momentary changes in the sitting position by movements of the buckled-up occupants on the seat.